Thermionic feed-back amplifier



April 2, 1946.

A; H. ROCHE ETAL THERMIONIC FEEDBACK AMPLIFIER Filed Nov. 24, 1942 2Shets-Sheet 1 A ia-,2, 1946.

,A. H. ROCHEET AL 2,397,625

THERMIONIC FEEDBACK AMPLIFIER Filed Nov. "24, 1942. 2 Sheets-Sheet '2 4rORWEY Patented A e-z, 1946 THEBMIONIC FEED-BACK AMPLIFIER Alleman HollyRoche and Henry Gerald Kidner,

London W. C. 2, England, assignors to International Standard York, N. Y.

Electric Corporation, New

Application November 24, 1942, Serial No. 466,820 In Great Britain March10, 1942 3 Claims. (01. 179-111) This invention relates to negativefeedback amplifiers of the type in which. the gain is adiusted byvarying the loss in the feedback path.

Many amplifiers used in communication systems are required to have somemeans ofcontrolling the gain. In feedback amplifiers this control may beentirely extemal to the ampli- Difficulties are, however, oftenencountered with instability as regards oscillation in amplifiers havingav large amount of feedback, owing to the difliculty of maintaining thenecessary high frequency correction as the gain is changed. Thus, if thecorrection is sufiicient to prevent oscillation with the maximum amountof feedback, it usually affects the gain of the amplifier in the rangeof frequencies it is intended to transmit when the feedback is reducedto increase the gain. Alternatively, if the correction is such that thetransmitted frequencies are unaffected when the feedback is a minimum,it proves insufficient to prevent self-oscillation with maximumfeedback.

The present inventionprovides a means for overcoming these difficulties,whereby the, advantage of decreased harmonic content which accrues fromthe reduction in gain by means of increased feedback may be retained,while stability as regards self-oscillation remains substantially thesame for all gain settings.

The method of the invention consists essentially in providing theamplifier with two feedback paths, one of which embraces any number ofstages including the last stage but excluding the first, and the otherembraces all the stages. Means is provided in both feedback paths forembracing all the stages and a local feedback path embracing any numberof stages excluding the first and including the last, means beingprovided for simultaneously increasing the gain of the forward pathcomprising all the stages, and reducing the loss in the over-allfeedback path, in such manner that the propagation round the over-allfeedback loop remains substantially constant for all settings of thesaid means.

According to another aspect, the invention comprises a multi-stagenegative feedback thermionic vaive'amplifier having an over-all feedbackpath, comprising means for simultaneously .decreasing the gain of theforward amplifying path and decreasing the loss of the feedback path bysubstantially equal amounts in such manner that the harmonic distortionintroduced by the amplifying path decreases as the gain is decreased.

According to a further aspect, the invention resides in a multi-stagenegative'feedback thermionic valve amplifier comprising a forwardamplifying path which includes a local feedback path, and an over-allfeedback path, each of the said feedback paths having a variable loss,means- .from the following detailed description of an embodiment, whichrefers to the aocompanyin drawings in which:

Fig. 1 shows a block schematic circuit diagram of the arrangement of anamplifier according to the invention;

Fig. 2 shows adetailed schematic circuit diagram of the amplifier; and

Fig. 3 shows a circuit diagram of the gain control means which operatessimultaneously in both the feedback paths. v

For convenience in describing the invention. the embodiment shown inFig. 2 is a two-stage amplifier in which the local feedback pathembraces the last stage only. The inventionis how.-

adjusting the attenuation of each path, the separate'means being coupledfor simultaneous adjustment so that the gain of the forward path of thewhole amplifier is changed at the same time as the loss in the overallfeedback path in such a way that the total feedback p) remainssubstantially constant.

According to the invention, there is provided a multi-stage negativefeedback thermionic valve amplifier comprising an over-all feedback pathever applicable to amplifiers with anylarger number of stages, and thelocal feedback path may embrace any number of other stages besides thelast, not including the first stage. I

In accordance with the usual practice in considering feedbackamplifiers, thecircuit is regarded as comprising a forward amplifyingpath and ,a feedback path connected in a loop called the feedback loop;the voltage transfer ratio of the forward path is denoted by u. and thatof the feedback path is denoted by p; the. produce 14}! and,

represents the voltage transfer ratio or propagation' constantcorresponding to a transit .once

I round the feedback loop. a is greater than 1 and B is less than 1, andthe product s is generally made very large compared with 1 in order toobtain the well known advantages of negativefeedback.- This 'productdenotes what is called the totalfeedback, and the gain round thefeedback loop is determined by log (m8); It is, however, not-essentialfor the purposes of the invention that the product 43 should be verylarge.

It is well known that the effective gain of the amplifier when thefeedback is operating is practically given by 1/5 when 13 is lar e. Theeffective gain may therefore be reduced by increasing ,8, whichincreases the total feedback, provided the conditions of stability arealways satisfied. However, as the total feedback is increased, themargin of stability tends to become progressively lowered, and itbecomes very dificult to design a variable feedback path which givesadequate correction'when the total feedback is large, and also freedomfrom distortion in the range of transmitted frequencies when the totalfeedback is small. According to the present invention, it is arranged sothat when p is increased for the purpose of reducing the effective gainof the amplifier, the value of a is at the same time decreased byincreasing the feedback of energy in a local feedback path, in suchmanner that the product 5 remains practically constant in magnitude andphase.

This method not only allows a large variation of the effective amplifiergain to be made while at the same time maintaining the'same margin ofstability, but also it reduces the harmonic coni tent of the output byvirtue of the local feedback.

In Fig. l, the outline A represents the amplifier comprising a forwardamplifying path it enclosed in the outline B, and a feedback path a. Theforward amplifying path includes a number of stages denoted by in andother stages a: enclosed in the outline C and provided with a localfeedback path 52. In the following equations the harmonic distortion theas stages when neither the p nor the a: feedthe harmonic distortion ofthe a path as above defined, when the p. feedback path is not operating,is denoted by d.

Then it follows that:

If no is the voltage transfer ratio corresponding to the effective gainof the complete amplifier A, then 7 awn/ mmune is varied in ordertoproduced at the output of back paths are operating is denoted by D,and

and if s is kept. constant by varyir g 3 v at the same time at ,8, thevalueof the harmonic con tent do decreases as the effective gain isdecreased. If 23: is also large compared with 1, this decrease will bein exactly the same ratio as the decrease in n; otherwise itwill beslightly less. These are the principles on which the invention is based.

The simultaneous variation of ,8. and p: is pref erably achieved 'bymechanically coupling the controls, though any other couplingarrangementmay be used-instead, if desired. The corresponding attenuating networkswill be unavoidably made more complicated by the new requirement,

and the degree of complication will depend uponthe degree of constancyrequired for as.

Fig. 2 shows. the details-of a practical embodimentof the invention inaccordance with Fig. 1

- comprising a two-stage amplifier.

The signals arriving from the line are applied to the input transformerT1, which is a threewinding transformer or hybrid coil of a kind wellknown in the art. The signals reach the control grid of the first valveV1, which may be of the pentode class.

After amplification plied to the control grid of the second valve V2,

which may also be a pentode, and thence through a balanced output hybridcoil T2 to the outgoing line.

The plate and screen grid of V1 are respectively decoupled by resistanceR7 and condenser Ca, and by resistance R6 and condenser C1; and thescreen grid of V2 is decoupled by resistance RB and condenser C4. Thehigh tension supply for the plate of V2 is fed through the highimpedance winding of the hybrid coil T2, a by-pass condenser Ce beingconnected across the supply. The grid bias potential for V1 is obtainedby means of the cathode circuit resistance R4 shuntedlby the condenserCr, and the grid bias for V2 is obtained from the potential drop acrosspart of the cathode circuit resistance R15 applied through the gridresistance R5. The interstage coupling network comprises the inductanceL1 and the resistance R3 in series with the plate circuit of V1, and thecondenser Ca connecting the plate of Vi and the grid of V2. A portion ofthe energy of the output signals is fed back through the over-all or 5feedback path to the input of the amplifier. This'path is connectedbetween thetwo hybrid coils as shown so as to be conjugate respectivelyto the input and output lines at its ends, for which purpose thebalancing resistances R1 and R2 are provided. This feedback pathcontains in order starting from T2: an impedance changing network ofthree resistances R9, R10 and Ru, which is also used for making theconnection between the balancedhybrid coil T2 and the rest of thecircuit, which is unbalanced; part of the gain'control attenuator GA,comprising a variable bridged T type of attenuating network; and a fixednetwork of resistances Ru, R1: and R14 and condensers C1 and C. The

' variable network comprises fixed series resistances R35 and R20 avariable shunt resistance .Ra-l,,'and a variable bridge resistance R21shunted by a variable condenser C11. All these variables aremechanically coupled. The fixed network is provided with the condensersC1 and Co for the pur-' pose of providing feedback loop correction-atthe lower and upper frequencies, respectively. I

The local or p: feedback path for the valve V2 comprises an adjustableresistance R1 connected in series with the cathode, a variable portionof which is shunted'byasmall condenser Cs. This.

network is designed to give the desired magnitude in V1, the signals areand phase of the feedback for the valve V: at all settings of the gaincontrol attenuator in the p path, by arranging that the condenser Cshall substantially short-circuit (for the signal frequencies) avariable portion of R15, the control of which is mechanically coupled tothe controls of the other attenuator. As the effective value of R15 isprogressively reduced, the effect of the small condenser C5 in shuntwith it decreases, so

that both the phase and the magnitude of the ,8: feedback aresimultaneously changed.

A further control over the phase shift at high frequencies is providedby the condenser On. This is varied at the same time as the networkresistances, and enables a more desirable phase characteristic to beobtained. In practice it is found that only two or three differentvalues of this condenser are necessary in the case, for example, of anattenuator giving ten 1 db steps. Obviously, any theoretically possibledegree of correction may be obtained if the circuit is allowed to becomesufficiently complicated.

Figure 3 shows the details of one form ofthe gain control attenuator GAof Fig. 2. The terminals of the attenuator are similarly numbered inboth figures. It comprises four rotatable switches SI, S2, S3 and S4,which are mechanically coupled, and each has eleven steps giving a rangeof attenuation of 0 to db, for example. The first three rotors SI, S2,and S3, control the bridged T network in the ,8 path, and respectivelyadjust R37, R21 and Cu. The rotor S4 controls the resistance R in the p:path. The steps of the switch SI have been marked with the correspondingrelative effective amplifier gain in decibels in accordance with theabove assumed range, and it will be noted that the gain will be reducedby turning the switch anti-clockwise.

V The variable condenser C11 comprises three separate fixed condensersCilA, Cup and C110 one of which is shunted across R21 as required,according to the position of the switch. As already explained, thecapacities of these condensers are chosen to give a sufficient amount ofphase correction at frequencies above the pass range of the amplifierfor each setting of the gain control. The rotor S4 is shunted by thefixed condenser C10 and when it is adjusted, it changes both themagnitude and the phase of the gain in the a path.

' The fixed series resistances of the bridged T network R25 and R26 areshown connected to the contact arm of the'switch SI between terminals Iand 3.

There is, of course, no reason to limit the attenuator to the preciseexample which has been described in connection with Fig. 3, whichrepresents an actual case which has proved satisfactory in practice. Itcould obviously have other numbers of steps, and be designed to cover a,different attenuation range, and any other known type of adjustingswitch or arrangement could be used.

of course have to be designed accordingly.

The invention is likewise not intended to be limited to the relativelysimple networks which have been described used for the ,8 and #2 paths.Th networks may be as complicated as desired. according to thestringency of the requirements which have to be met, and the details ofthe adlusting arrangements described in Fig. 3 will It will usually befound, however, that itv is convenient to allow a small variation in thevalue of p from the theoretical constant value at the different gainettings in order. to obtain some simplification of the networks.isstated that #5 is substantially constant, it is to be understood tocover such deliberately allowed small variations.

Finally, the invention is not confined to the particular two-stageamplifier shown in Fig. 2. It is applicable to any negative feedbackamplifier having two or more stages, and persons skilled in the art willknow how to make the necessary modifications to adapt the invention toother types of feedback amplifier.

What is claimed is:

1; A multi-stage negative feedback vacuum tube amplifier including'abalanced hybrid coil circuit coupled to the input end of said amplifier,a second balanced hybrid coil circuit coupled to the output end of saidamplifier, a forward amplifying path from the input to said amplifiergoing through said amplifier to the output thereof, a resistance couplednegative feedback path from the output end of the amplifiergoing back tothe input end of said amplifier, a second resistance coupled-negativefeedback path going from the output end of the amplifier back to theinput of the second stage, and means for simultaneously adjusting theloss in each feedback path so that the over-all loop gain remains subatwo-stage amplifier having a forward ampli-- tying path is provided, anegative feed back in the cathode circuit of the tube in the secondstage, a negative feedback from the output of the second stage to theinput of the first stage, and means for simultaneously adjusting theloss ineach feedback path so that the over-all gain in the two-stageamplifier remains substantially constant in magnitude and phase.

ALLEMAN HOLLY ROCHE.

HENRY GERALD KIDNER.

Thus, when it

